538 TRANSACTIONS OF SECTION A. 



2. The Magnetic Field produced by the Motion of a Charged Condenser 

 through Space. By W. F. G-. Swann, B.Sc. 



Alter referring to the impossibility of detecting the magnetic field by means 

 of a compass needle, the possibility of detecting it by means of a rotating coil 

 was discussed. It was first shown that in the case of a closed circuit at constant 

 potential moving through space in company with a system of charged bodies, 

 the total magnetic flux through the circuit is zero. If, however, the space within 

 the coil is partially filled with a dielectric, then, provided that the S.I.C. may 

 be looked upon as a quantity absolutely continuous throughout the dielectric, 

 a magnetic flux through the coil should exist; it was shown further, however, 

 that if dielectric action is to be explained entirely by the presence of electric 

 charges, or doublets, no resultant magnetic flux is to be anticipated, even in this 

 case. 



Two condensers were formed from three parallel plates, the middle plate 

 being common to both condensers. Two coils fastened to the same axle, and 

 wound in opposite directions so as to eliminate fluctuation of the earth's field, 

 were rotated, one between the plates of each condenser. The apparatus gave a 

 null effect, which is taken to support the doublet theory of dielectric action. 



3. On the Secondary Radiation from Carbon at Low Temperatures. 



By V. E. Pound. 



In this paper a series of experiments were described, in which measurements 

 were made on the secondary rays from carbon in air when bombarded in a high 

 vacuum by the Alpha rays from polonium. 



Observations were made (1) at room temperature and (2) at the temperature 

 of liquid air. The secondary radiation excited at the lower temperature was 

 found to be about 50 per cent, greater than that obtained at the higher tempera- 

 ture, and this marked difference was ascribed to the presence of the large 

 quantity of air which is known to be occluded in the carbon. The same pheno- 

 menon was observed when hydrogen was occluded in the carbon instead of air. 



4. Photoelectric Fatigue. By H. Stanley Allen, M.A., D.Sc. 



The observation of Hertz in 1887 that the electric spark passes more readily 

 when the spark gap is illuminated by ultra-violet light led to the discovery by 

 Hallwachs of the photoelectric current. A negatively charged body often loses 

 its charge rapidly when exposed to light, especially to ultra-violet light. The 

 discharge is due to the emission of negative electrons from the illuminated 

 surface. The photoelectric activity of a freshly polished metal surface diminishes 

 with the time, falling off rapidly at first, more slowly later on. This is known 

 as the ' fatigue ' of the Hallwachs effect. In the early literature of the subject 

 the fatigue was attributed to the direct action of the light, but Hallwachs showed 

 in 1904 that fatigue proceeds in complete darkness, so that light cannot be the 

 primary cause of the change. This result has been questioned, but is now corf 

 firmed by the experiments of Bergwitz. Dember, Ullmann, and Allen. Light can, 

 however, set up secondary actions tending to accelerate or retard fatigue. 



Hallwachs has shown that at ordinary pressures fatigue is more rapid in a 

 large vessel than in a small one. This also was called in question by Aigner, but 

 is confirmed by Ullmann and by Allen. 



The fatigue is practically independent of the electrical condition of the plate. 

 This was shown by v. Schweidler and has received confirmation in the researches 

 of Hallwachs, Sadzewicz, and Allen. 



Experiments in a vacuum have led to contradictory results. Lenard and 

 Ladenburg found marked fatigue in certain cases, while for the alkali metals, 

 first investigated by Elster and Geitel, several observers conclude that there is 



